The present invention relates to an apparatus for regenerating original signals, and particularly to an apparatus for regenerating original signals while compensating missing portions that develop in the waveforms when digital signals are to be recorded, regenerated or transmitted.
Digital signals that are to be recorded or regenerated by a VTR (video tape recorder) have to be transmitted or received via a transformer, resulting in a cut-off in the low-frequency band. Further, when the signals are transmitted and received via a circuit having resistance R and capacity C, such as a telephone circuit, the signal amplitude is attenuated due to the transient phenomenon CR. Therefore, cut-off takes place not only in the low-frequency band, but also in the high-frequency band as well as in the intermediate-frequency band. Among the digital signals, furthermore, codes of the NRZ series develop a conspicuous sag in amplitude to the cut-off of low frequencies, often making it difficult to distinguish "1" from "0".
It is therefore necessary to regenerate original signals while compensating missing portions in the waveforms. For this purpose, there has been proposed a method which is based upon the quantized feedback (e.g., IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. COM 22, No. 1, JANUARY 1974, "Quantized Feedback in an Experimental 280-Mb/s Digital Repeater for Coaxial Transmission", F.D. WALDHAUER).
Construction and operation of the above method are briefly described below with reference to FIGS. 1 and 2.
The quantized feedback method consists, as shown in FIG. 1(a), of a decision circuit 12 made up of a comparator 2 and a latch circuit 3, a feedback circuit having a lowpass filter 4, and an adder 1. It is now presumed that an original signal g(t) generated from a signal source 20 passes through a system 21 which cuts off a predetermined frequency. This system may be, for instance, a recording/ regenerating system or a transmission system. It is presumed that this system has low-frequency cut-off characteristics P(f), and the signal which has passed through this system is regarded to be an input signal i(t). The fundamental operation of the quantized feedback method is to regenerate, from a signal 0(t) produced by the decision circuit 12, a signal s(t) of low-frequency component lost through the recording/regenerating system 21. That is, the decision circuit 12 consisting of comparator 2 and latch circuit 3 determines the output i(t) +s(t) of the adder 1 to be "1" or "0", and produces a signal 0(t) similar to the original signal g(t). The signal 0(t) is then passed through a low-pass filter 4 to extract the signal s(t) of a predetermined low-frequency component. If the filter 4 has transmission characteristics Q(f) as shown in FIG. 1(c) which has the following relation, EQU Q(f)+P(f)=1 (1)
and if the error rate of signal 0(t) after having been determined is sufficiently small, the filter 4 regenerates output s(t) which corresponds to the original low-frequency component. The thus regenerated low-frequency signal s(t) is added to the input signal i(t) through the adder 1 to regenerate the signal 0(t) which is similar to the original signal g(t).
FIG. 2 shows signal waveforms at each of the above-mentioned portions.
According to the above quantized feedback method; however, any error that occurs in the code may undergo chain propagation and may not be corrected. The reason is attributed to a positive feedback loop that is formed in the circuit by the comparator 2 and filter 4 as is obvious from the construction of FIG. 1(a). Described below is the condition where error in the code undergoes chain propagation. Here, the amplitude of the signal produced by the adder 1 is standardized to 1.
It is presumed that i(t.sub.1)+s(t.sub.1)&gt;1/2 at a moment t.sub.1 in FIG. 2. At this moment, it is further assumed that noise n(t.sub.1) is added to the signal i(t.sub.1) or s(t.sub.1), and the signal produced by the adder 1 becomes smaller than the decision level 1/2. Namely if i(t.sub.1)+s(t.sub.1)+n(t.sub.1) &lt;1/2, the output 0(t) of the decision circuit 12 changes from "1" to "0" at the moment t.sub.1, and the code error continues up to a moment t.sub.2 at which the output 0(t) should have changed to "0". The same also holds true even when an error causes the signal level to change from "0" to "1". Such a phenomenon occurs even when the amplitude of the input signal i(t) is changed for some reason. That is, under the condition where i(t.sub.1)+s(t.sub.1)&gt;1/2, the signal s(t.sub.1) may change to s'(t.sub.1) so that i(t.sub.1)+s'(t.sub.1)&lt;1/2. Then, the output signal 0(t) of the decision circuit 12 changes from "1" to "0", and an error occurs in the code as in the above-mentioned case.